It has become fairly common practice in the design of electronic tone generators to employ resistor-capacitor circuits, or networks, (RC networks) to amplitude modulate the tone signals. The rapid charge of a capacitor and its subsequent discharge through a resistor produces a decaying voltage waveform similar to tones produced by musical instruments.
U.S. Pat. No. 4,273,019, granted June 16, 1981, to M. Goto for "Electronic Tone Generator" and U.S. Pat. No. 4,545,279, granted Oct. 8, 1985, to T. Sano for "Electronic Music Note Generator", disclose tone generators employing RC networks for shaping tone waveforms.
Various forms of electroacoustic devices have been employed to convert the shaped tone waveforms to audible sound.
Piezoelectric crystal devices, often called "piezo-buzzers", have proven themselves to be particularly capable of producing high quality musical tones similar to those produced by natural musical instruments.
Piezo-buzzers are a.c. voltage driven devices which must be isolated from any direct current signals in the tone generator circuitry. In the past this isolation has been accomplished by imposing a capacitor in the circuit to the piezo-buzzer. But the incorporation of a capacitor is not always practical in tone generators fabricated as an integrated circuit. And the use of integrated circuits is highly desirable for tone generators because of the low cost and reliability of the integrated circuits. Capacitors usually must be formed as distinct elements apart from the integrated circuit chips and the chip must be provided with bonding pads by which the capacitor is connected to the circuit. Such pads use up valuable space on the chip.
Other prior art tone generators utilizing logic techniques for forming the envelope of the tone signal have employed digital to analog converters to produce a waveformed current signal. This signal has been coupled to the piezo-buzzer through a tone signal input. Although no blocking capacitor is required for such a circuit, the digital to analog converter and logic circuitry greatly complicates the tone generator.
Because of the low voltage supply available for IC tone generators, usually of the order of three volts and rarely more than six volts, it is usually desirable to amplify the signal supplied to the piezo-buzzer. U.S. Pat. No. 4,567,806, granted Feb. 4, 1986 to M. Kodaira for "Sound Generator" and No. 4,487,099, granted Dec. 11, 1984 to H. Ebihara for "Electro-Acoustic Transducer Drive Circuit for Producing Damped Waveform Envelope Musical Notes", disclose the use of impedance coils for stepping up the signal voltage to the piezo-buzzer. Coils, like capacitors, are simply not economically compatible with integrated circuit devices.
Another circuit for amplifying a tone signal is disclosed in U.S. Pat. No. 4,068,461, granted Jan. 17, 1978 to J. R. Fassett, et al. for "Digital Electronic Alarm Watch". This patent discloses the use of MOS inverters to drive a piezoelectric device which produces a watch alarm signal. The patent contains no disclosure, however, of how such inverters might be utilized in a shaped waveform tone generator.
There continues to be a need for improved circuitry in a shaped waveform tone generator which provides push-pull amplification of tone signals imparted to a piezo-buzzer and which can be accomplished in a fully integrated circuit.